A MRI apparatus measures an NMR signal generated by a nuclear spin that comprises tissues of an object, particularly a human body to image forms and functions of the head, belly, limbs, etc. two-dimensionally or three-dimensionally. In imaging, a phase encode that varies depending on the gradient magnetic field as well as frequency-encoding are provided to an NMR signal, the NMR signal is measured as time-series data. The measured NMR signal is reconstructed into an image by performing a two-dimensional or three-dimensional Fourier transform.
In an ultra-high magnetic field MRI apparatus of 3 T or more, static magnetic field inhomogeneity that is spatially inhomogeneous in a static magnetic field increases. On the other hand, an SSFP (steady-state free precession) sequence that repeats measuring an echo signal in a short time with magnetization in a steady-state free precession state and an EPI (Echo Planar Imaging) sequence that measures a plurality of echo signals using a single or multiple excitations by oscillating a readout gradient magnetic field are sensitive to static magnetic field inhomogeneity. Therefore, in an image obtained using these pulse sequences in an ultra-high magnetic field MRI apparatus of 3 T or more, artifacts generated based on static magnetic field inhomogeneity are conspicuous. Also, a pencil-beam-shaped excitation using a two-dimensional excitation may deteriorate an excitation profile due to static magnetic field inhomogeneity.
Taking an SSFP sequence as an example, influence of static magnetic field inhomogeneity will be described. In the SSFP sequence, assuming RF pulse repetition time as TR, band artifacts occur in an image at a frequency where static magnetic field inhomogeneity is an odd multiple of ½ TR. When the SSFP sequence is used for cineradiography of a heart, in a 1.5 T apparatus, band artifacts occur at a position where there are no diagnostic problems in an image. However, because static magnetic field inhomogeneity increases in a 3 T apparatus, band artifacts may overlap with a cardiac short axis where band artifacts are regions of interest. Consequently, evaluation of cardiac functions such as ejection fraction may be influenced.
As a standard method to reduce static magnetic field inhomogeneity, Bo shimming that applies appropriate current to a shim coil to adjust static magnetic field homogeneity in an imaging target region is used. In particular, if static magnetic field homogeneity is increased locally, the method called local Bo shimming that generates a compensation magnetic field to reduce local static magnetic field inhomogeneity is used. Specifically, by measuring static magnetic field inhomogeneity unique to the apparatus in a case where nothing exists in an imaging space when an MRI apparatus is installed and then measuring static magnetic field inhomogeneity in an imaging space including an imaging target when imaging is performed, a compensation magnetic field that reduces static magnetic field inhomogeneity in an imaging target region using both the data is generated in a shim coil.
However, band artifacts of an SSPF sequence cannot be solved only by local Bo shimming. Therefore, in the SSPF sequence, in addition to local Bo shimming, a method that slightly shifts an excitation frequency of an RF pulse so that band artifacts do not overlap with a heart region is used. As a method to calculate the excitation frequency, a method that creates a static magnetic field inhomogeneity estimate map from static magnetic field inhomogeneity after local Bo shimming and calculates an excitation frequency to reduce average static magnetic field inhomogeneity in the cardiac short axis direction is used. Alternatively, as a preliminary measurement, the method (PTL 1) is used where imaging is performed to create a plurality of images with the excitation frequency slightly shifted, and an operator visually searches for an appropriate excitation frequency so that artifacts do not overlap on the short axis of the heart.